If materials such as metals (e.g. steel) or components made of these materials are exposed to corrosive environmental conditions, e.g. as a result of acids/bases and/or as a result of an electric voltage, they can be attacked by the corrosive environmental conditions, which can lead to impairment of their function. However, some materials form an inert (sluggishly reacting) passivating layer which inhibits, i.e. slows or prevents, corrosion of the material underneath. These materials can be used in a targeted manner in order to increase the chemical resistance of components to corrosive environmental conditions as occur, for example, in batteries or rechargeable batteries. For example, a material having a suitable composition can have been or be provided (e.g. by mixing chromium into it) so that a stable passivating layer (e.g. an oxide layer such as chromium oxide) is automatically (naturally) formed on the material. However, the passivating layer at the same time impairs the electrical conductivity on the surface of the material or of the component. In other words, the surface resistance thereof is increased by the passivating layer and electrical contacting of the material or component is therefore made more difficult.
In the case of materials or components which are used in electrical appliances (e.g. energy storages such as rechargeable batteries, batteries or capacitors) and are employed, for example, for contacting or for conducting the electric current, the provision of an electrical contact (i.e. illustratively a low interfacial resistance) can require a low surface resistance in order to reduce resistive losses and thus impairment of the efficiency. For this reason, the natural passivating layer is frequently replaced by a synthetically produced electrically conductive protective layer which inhibits corrosion but has a lower surface resistance compared to the natural passivating layer.
In the case of thin foils (e.g. thinner than 200 microns) which are required, for example, for use in electrodes, e.g. metal foils composed of copper or aluminum, most of the conventional processes for producing synthetic protective layers lead to damage to the foils, since their mechanical strength is greatly reduced by the low thickness. Thin foils are illustratively very sensitive and cracks or holes can remain in the foil.
Processes which avoid damage to the foils customarily utilize a wet-chemical deposition method in which particles (e.g. flocs or microparticles having sizes in the micron range) are dispersed in an organic solvent and applied together with the organic solvent to the foil. The solvent is subsequently vaporized by means of heat, so that the particles remain and form a protective layer. However, the organic solvent can, in this case, react chemically with the foil so as to form an electrically insulating layer between the protective layer and the foil, which inhibits the flow of current and thus restricts the ability of wet-chemical deposition methods to reduce the surface resistance.
In addition, no chemical bonds are formed between the particles and the surface of the foils (these adhere, for example, only by means of van der Waals interactions, which are weak chemical bonds), as a result of which both the electrical and mechanical properties of the protective layer (e.g. adhesion of this to the foil) are impaired. In other words, the protective layers formed by particles cannot be loaded mechanically and are easily damaged, which leads to corrosion of the underlying foil. Modern energy storages which provide high electrical (cell) voltages (e.g. greater than 4 V) require electrodes having a high chemical resistance.
For example, carbon is customarily applied in particle form to the foils. However, the layer thicknesses which can be achieved by this method are in the range of microns (μm), i.e. these are illustratively very thick. Since the ohmic resistance of the layer increases with increasing layer thickness, the interfacial resistances (ICR=interfacial contact resistance, also referred to as interfacially induced resistances) generated thereby result in large resistive losses and thus impair the efficiency.
As an alternative, carbon is applied in floc form (known as “carbon flakes”) to the foils. In this way, it is possible to achieve significantly smaller layer thicknesses, e.g. a few nanometers (nm), which reduces resistive losses. However, a porous layer which has many openings at which the foil is exposed is produced thereby. There are illustratively regions of the foil on which there are no flocs to protect the foil against corrosion. The exposed regions can be corroded further by further wet-chemical manufacturing processes, e.g. during application of active materials for electrodes (anodes or cathodes) in rechargeable batteries, as a result of which the interfacial resistance is increased, e.g. by oxide formation.